Method of making resistance element

ABSTRACT

A method of making wire wound electrical resistance elements comprising the steps of winding a respective resistance wire helically around and along each of a plurality of elongated wire cores which cores are coated with an insulating material, coating each of said wound cores with a protective layer, arranging said wound cores in closely spaced parallel relation, encapsulating said arranged wire cores in a block of plastic material, cutting through the block and the wound cores at longitudinally spaced points along planes perpendicular to the longitudinal axes of the wound cores to form a plurality of wafers each of which contains a plurality of resistance elements, and then separating said resistance elements from the plastic of the respective wafers. The disclosure also includes the steps in which prior to separating resistance elements from the plastic of the wafer, the ends of the wire cores are contacted with an etchant to etch away a portion of the ends of the wire cores, after which the ends are coated with a film of an insulating material.

United States Patent [191 Rudd et al. a a

[ Nov. 26, 1974 METHOD OF MAKING RESISTANCE ELEMENT [75] Inventors:Floyd R. Rudd, North Wales;

George D. MacKenzie, Maple Glen, both of Pa.

[73] Assignee: TRW, Inc., Cleveland, Ohio [22] Filed: Dec. 4, 1972 [21]Appl. No.: 311,555

[52] US. Cl 29/610, 29/412, 29/417 29/423, 29/424, 338/263 [51] Int. ClH0lc 17/00 [58] Field of Search 29/610, 412, 417, 620,

[56] References Cited UNITED STATES PATENTS 3,252,205 5/1966 Hancock etal 29/424 X 3,358,362 12/1967 McElroy ..29/620X Primary Examiner-CharlesW. Lanham Assistant ExaminerVictor A. DiPalma Attorney, Agent, orFirm-Jacob Trachtman [57 'ABsTRACT A method of makingwire woundelectrical resistance elements comprising the steps of winding arespective resistance wire helieally around and along each of aplurality of elongated wire cores which cores are coated with aninsulating material, coating each of said wound cores with a protectivelayer, arranging said wound cores in closely spaced parallel relation,encapsulating said arranged wire cores in a block of plasticmaterial,cutting through the block and the wound cores at longitudinally spaced'points along planes perpendicular to the longitudinal axes of the woundcores to form a plurality of wafers each of which contains a pluralityof resistance elements;and then separating said resistance elements fromthe plastic of the respective wafers. The disclosure also includes thesteps in which prior to separating resistance elements from the plasticof the wafer, the ends of the wire cores are contacted with an etchantto etch away a portion of the ends of the wire cores, after which theerfds are coated with a film of an insulating material.

7 Claims, 7 Drawing Figures METHOD OF MAKING RESISTANCE ELEMENT Thepresent invention relates to an electrical resistance element and amethod of making the same. More particularly, the present inventionrelates to a wire wound resistance element for a potentiometer and abatch method for making the resistance elements.

The wire wound resistance elements used in potentiometers in general,comprise a core having an electrical resistance wire helically woundaround the core. One type of core used is a magnet wire, such as copper,having a coating of an insulating material thereover to insulate theresistance wire from the core wire. The resistance wire is generallycoated with a strip of an insulating material to hold the turns of theresistance wire in place on the core. These resistance elements aregenerally made by winding the resistance wire on an elongated length ofthe core. The insulating strip is then coated and cured on the woundcore. The wound core is then cut into the desired lengths for thepotentiometer elements, generally with a metal saw. This method has anumber of disadvantages. When the wound core is cut into the individualelements, the sawing with a metal saw results in unwinding theresistance wire at the ends of the elements and a smearing of theresistance wire across the ends of the metal core. The unraveled andsmeared ends of the resistance wire must be removed by hand labor usingtweezers. This slows down the operation and adds considerably to thecost of making the elements. Also, the insulating strips coated on thewound core tend to creep onto the wiper track area of the element duringthe application of the strip and could ruin the element.

It is therefore an object of the present invention to provide a novelwire wound resistance element for use in a potentiometer.

It is another object of the present invention to provide a wire woundresistance element in which the turns of the resistance wire are securedin position on the core particularly at the ends of the core.

It is a further object of the present invention to provide a novelmethod of making a wire wound resistance element.

It is a still further object of the present invention to provide amethod of making a wire wound resistance element without providing anyloose ends of the resistance wire or smearing the ends of the resistancewire across the ends of the core.

It is still another object of the present invention to provide a methodof making wire wound resistance element on a batch basis. These objectsare achieved by providing a resistance element in which the ends of awire core are spaced inwardly from the ends of an insulating layer onthe core, a protection layer is coated around the resistance wirewinding, and insulating films are coated on the ends of the wire coreand extend over the end portions of the protection layer. The resistanceelements are made by placing a plurality of elongated resistance wirewound cores in a plastic block and cutting the block into a plurality ofwafers each containing a plurality of the resistance elements. All ofthe resistance elements in a wafer are treated simultaneously to spacethe ends of the wire cores from the ends of the insulating layer thereonand to provide an insulating film on the ends of the wire cores. Theresistance elements are then separated from the plastic of the wafers.

FIG. 1 is a side elevation view, partially sectioned, of the wire woundresistance element of the present invention. 7

FIG. 2 is an enlarged perspective view of a portion of an elongated wirewound and coated core from which the resistance elements of the presentinvention are made and illustrates the first step in the method of thepresent invention.

FIGS. 3 7 are perspective views illustrating the various steps of themethod of the present invention.

Referring initially to FIG. 1, the wire wound resistance element of thepresent invention is generally designated as 10. Resistance element 10comprises a magnet wire core 12, such as of a copper wire, having alayer 14 of an electrical insulating material, such as a polyimideresin, coated on the surface thereof. The insulating layer 14 extendsbeyond both ends of the core 12. An electrical resistance wire 16 ishelically wound around the insulating layer 14 along the entire lengthof the insulatinglayer 14. A protection layer 18 of an electricalinsulating material, such as a polyimide or epoxy resin, is coated overthe resistance wire winding 16. The protection layer 18 has a narrowopening 20 therethrough which extends longitudinally along the entirelength of the resistance element 10. The opening 20 exposes a portion ofthe resistance wire winding 16 to provide a track along which a movablecontact of a potentiometer .can make contact with the resistance wirewinding. At each end of the resistance element 10, a thin film 22 of anelectrical insulating material, such as apolymide or epoxy resin, iscoated over each end of the core 12. Each of the insulating films 22extends across a respective end of the insulating layer 14 and onto thesurface of the protection layer 18.

To make resistance element 10, according to the method of the presentinvention, one starts with an elongated length of the magnet wire core12 having the layer 14 of the electrical insulating material thereon.The resistance wire 16 is helically wound around the elongated length ofthe insulated core 12 along the entire length of the core. Theprotection layer 18 is then applied to the wound core, such as byspraying, painting, or dipping. The protection layer 18 is thenpartially cured by a short heating cycle. For example, a polyimide resinprotection layer can be partially cured by heating at a temperature of200C to 225C for 5 to 15 minutes.

FIG. 2 is an enlarged view showing an end portion of an elongated length24 ofthe resistance wire wound core with the protection layer 18thereon. The narrow longitudinal opening 20 is then formed in theprotection layer 18 along the entire length of the core so as to exposea portion of the resistance wire. This can be achieved either by buffingthe protection layer 18 with a narrow abrasive impregnated rubber beltor by a blast of bicarbonate soda powder. The elongated length 24 of thewound core is thencleaned and the protection layer 18 fully cured. Theelongated length 24 can be cleaned by ultrasonic cleaning in freon MT orwith methylene chloride or a detergent followed by ultrasonic cleaningin freon MT. The polyimide protection layer 18 can be then fully curedby heating at 200C for l to 4 hours. t

A plurality of the length 24 of the wound cores, each typically between8 and 12 inches long, with the lengths 24 arranged in closely spaced,parallel relation are then encased in a block 26 of a plastic materialas shown in FIG. 3. The plastic material of the block 26 is one which isrelatively inexpensive and which is controllably soluble in a solventwhich does not attack the material of the lengths 24 of the wound cores.Polyester resins have been found suitable for this purpose. However,epoxy, polyurethane, silicone and thermoplastic resins as well as suchwaxes as candle wax are also usable depending on the material used forthe protection layers 18 and the insulating layers 14 of the woundcores. The material of the block 26 is also preferably filled withparticles of a mineral, such as mica, glass beads or silica. As many as100 length 24 of the wound cores can be included in the block 26.

As shown in FIG. 4, the block 26 is then cut completely through atuniformly spaced points along its length, along parallel planes whichare perpendicular to the longitudinal axes of the lengths 24 of thewound cores. The cuts can be made with any suitable tool, such as arotating circular diamond saw. The cuts are spaced apart a distanceequal to the desired length of the resistance elements being produced,typically V2 to inch. Thus, the block 26 is divided into a plurality ofwafers 28 with each wafer containing a plurality of lengths 24' forproducing the resistance elements 10.

The exposed ends of the wire cores 12, at each end surface of each wafer28, is contacted with an etchant to etch away a portion of the ends ofthe wire cores 12 as shown in FIG. 5. For wire cores of copper, asuitable etchant may be ammonium persulfate, nitric acid or ferricchloride. As shown in FIG. 5, this leaves the ends of the insulatinglayers 14 projecting beyond the ends of the wire cores 12.

The wafer 28 is washed and rinsed in water for removing residualetchant. Each wafer 28 is then immersed in a suitable solvent for aperiod of time necessary to dissolve or soften the plastic material atthe surfaces of the wafers 28. As previously stated, the solvent is onewhich will slowly dissolve the particular plastic being used, but doesnot attack the materials of the wound cores. When the plastic is apolyester resin, methylene chloride has been found to be a satisfactorysolvent. Chlorinated solvents can be used for epoxy and silicon resins,alcohols or ketones for polyurethane, and various hydrocarbon solventsfor waxes. When the wafers 28 are removed from the solvent, they arewashed with water to remove the softened surface layer of the plasticand any of the solvent. This exposes a portion of the protection layer18 at each end of each of the lengths 24' as shown in FIG. 6. The amountof the protection layers 18 which are exposed will depend on the lengthof time that the wafers 28 are immersed in the solvent. For example,using methylene chloride as a solvent for a polyester resin, leaving thewafers 28 in the solvent for approximately 10 minutes will dissolvesufficient amount of the plastic to expose approximately 10 mils of theprotection layer 18 at each end of each of the lengths 24'.

As shown in FIG. 7, a thin film 22 of an heating. The material is coatedon each end of each of the wire cores 12 with the film extending overthe exposed dissolved so of the protection layer 18. The insulatingmaterial films 22 are cured by heatingThe wafers 28 are then immersedinthe solvent bath and left in the solvent until all of the plastic isdissolvedso as to separate the individual lengths 24 in the wafers.After the plastic is completely dissolved, the lengths 24' proving theindividual resistance elements 10 are removed from the solvent andwashed to remove the solvent. The resistance elements 10 are then readyto be used in potentiometer assemblies.

Although the method of the present invention has been described withregard to making straight resistance elements 10, it can also be used tomake resistance elements which are in the form of a helix. For helicalresistance elements, the elongated wound cores 24 are wound in a helixand a plurality of the helically wound cores 24 are molded in a plasticblock 26. The block is then handled in the same manner as previouslydescribed.

The method of the present invention has many advantages some of whichare the following:

1. It allows large batch processing of the resistance elements so as toreduce the per unit cost of manufacturing the resistance elements.

2. It eliminates the need of peeling back by hand the loose ends of theresistance wire of each of the resistance elements, since the resistancewire is firmly held in place by the protection layer and the plasticblock when the wound core is cut into the individual elements.

3. The insulating film on each end of the wound core insulates the wirecore so as to prevent the resistance wire from shorting across the endsof the wire core, and

also helps hold the ends of the resistance wire in place.

4. The buffing operation which exposes the resistance wire path improvesthe noise characteristics of the resistance element.

5. By cutting the wafers from a large block, all of the resistanceelements in each wafer are of the same length.

Thus, the method of the present invention provides for the massproduction of the resistance elements 10 with greater ease of handlingthe resistance elements, with greater speed, at a lower cost perresistance element and with uniformity of size. Also, it provides aresistance element in which the resistance wire is firmly held in placeon the core and will not short out across the ends of the wire core.

We claim:

1. A method of making electrical resistance elements comprising thesteps of winding a respective resistance wire helically around and alongeach of a plurality of elongated wire cores which cores are coated withan insulating ma terial,

coating each of said wound cores with a protection layer,

arranging said wound cores in closely spaced parallel relation,

encapsulating said arranged wound cores in a block of plastic material,

cutting through said block and the wound cores at longitudinally spacedpoints along planes perpendicular to the longitudinal axes of the woundcores to form a plurality of wafers each of which contains a pluralityof resistance elements, and

then separating said resistance elements from the plastic of therespective wafers.

2. The method in accordance with claim 1 in which prior to separatingthe resistance elements from the plastic of the wafers, the ends of thewire cores are contacted with an etchant to etch away a portion of theends of the wire cores.

3. The method in accordance with claim 2 in which after the ends of thewire cores are etched away the ends are coated with a film of aninsulating material.

4. The method in accordance with claim 3 in which prior to coating theetched ends of the wire cores, a portion of the surface of each wafer isremoved to expose the end portions of the protection layer on eachresistance element, and the films of the insulating material coated onthe etched ends of the wire cores are also coated over the exposed endportions of the protection layers.

5. The method in accordance with claim 4 in which the portion of theplastic of each wafer is removed by immersing the wafers in a solventwhich dissolves the plastic but does not affect the materials of theresistance elements.

6. The method in accordance with claim 1 in which prior to assemblingthe wound cores in parallel relation, a narrow opening is formed in theprotection layer longitudinally along the entire length of the woundcore to expose a track of the resistance wire winding.

7. The method in accordance with claim 6 in which the opening in theprotection layer is formed by buffing the protection layer.

1. A method of making electrical resistance elements comprising the steps of winding a respective resistance wire helically around and along each of a plurality of elongated wire cores which cores are coated with an insulating material, coating each of said wound cores with a protection layer, arranging said wound cores in closely spaced parallel relation, encapsulating said arranged wound cores in a block of plastic material, cutting through said block and the wound cores at longitudinally spaced points along planes perpendicular to the longitudinal axes of the wound cores to form a plurality of wafers each of which contains a plurality of resistance elements, and then separating said resistance elements from the plastic of the respective wafers.
 2. The method in accordance with claim 1 in which prior to separating the resistance elements from the plastic of the wafers, the ends of the wire cores are contacted with an etchant to etch away a portion of the ends of the wire cores.
 3. The method in accordance with claim 2 in which after the ends of the wire cores are etched away the ends are coated with a film of an insulating material.
 4. The method in accordance with claim 3 in which prior to coating the etched ends of the wire cores, a portion of the surface of each wafer is removed to expose the end portions of the protection layer on each resistance element, and the films of the insulating material coated on the etched ends of the wire cores are also coated over the exposed end portions of the protection layers.
 5. The method in accordance with claim 4 in which the portion of the plastic of each wafer is removed by immersing the wafers in a solvent which dissolves the plastic but does not affect the materials of the resistance elements.
 6. The method iN accordance with claim 1 in which prior to assembling the wound cores in parallel relation, a narrow opening is formed in the protection layer longitudinally along the entire length of the wound core to expose a track of the resistance wire winding.
 7. The method in accordance with claim 6 in which the opening in the protection layer is formed by buffing the protection layer. 